1. Field of the Invention
The present invention relates to a lighting device in which a light emitting element is mounted in a package material using a glass substrate.
2. Description of the Related Art
In recent years, an electronic component using a glass package has been put to practical use. A glass material prevents moisture or contaminants from entering from the outside and attains high airtightness. The glass material is close in thermal expansion coefficient to a silicon substrate in which a semiconductor element is formed. Therefore, reliability of bonding is improved in case that the semiconductor element is mounted in the glass package. Moreover, the glass material is low in cost, and hence an increase in product cost may be suppressed.
FIG. 19 schematically illustrates a cross sectional structure of an LED lighting device in which an LED element is mounted in a glass material. Such a structure is disclosed in, for example, FIG. 1 of JP 2007-042781 A (hereinafter, referred to as Patent Document 1). As illustrated in FIG. 19, through-electrodes 52 are formed in a glass substrate 51. Electrode metallizations 53B for connecting are formed on the through-electrodes 52. A plurality of LED elements 56A are mounted on the electrode metallizations 53B. Upper surfaces of the LED elements 56A are electrically connected to one of the electrode metallizations 53B through wires 57. Electrode metallizations 53A for external connection are formed on a lower surface of the glass substrate 51. The electrode metallizations 53A are electrically connected to the through-electrodes 52. Therefore, power may be supplied to the LED elements 56A from the electrode metallizations 53A formed on the lower surface.
A Si substrate 54 formed with a through hole 58 is provided on an upper surface of the glass substrate 51 so as to surround the LED elements 56A. The Si substrate 54 is anodically bonded to the upper surface of the glass substrate 51. The Si substrate 54 has an inclined inner wall surface. A reflective film 55 is formed on the inner wall surface. Light emitted from the LED elements 56A are reflected on the reflective film 55 and emitted as light having directivity in an upward direction. The plurality of LED elements 56A are mounted, and hence a light emission intensity may be increased. Heat generated from the LED elements 56A may be radiated to the outside through the through-electrodes 52 and the electrode metallizations 53A.
The through-electrodes 52 of the glass substrate 51 are formed as follows. That is, an inner wall of the through hole formed in the glass substrate 51 is plated with Cu or Ni, and then the through hole is filled with a conductive resin or solder. The electrode metallizations 53A located on the rear surface (lower surface) of the glass substrate 51 are formed as follows. A Ti layer is deposited on a surface of a glass material by sputtering or evaporation. A Pt layer or a Ni layer which serves as a barrier layer for protecting the Ti layer is deposited on the Ti layer by sputtering or evaporation. Then, an Au layer for protecting surface oxidation is deposited by sputtering or evaporation. The resultant layers are patterned by a photo process.
FIG. 20 schematically illustrates a lighting device 60 in which an LED light emitting element 61 is embedded in a glass material. Such a structure is disclosed in, for example, FIG. 1 of JP 2007-306036 A (hereinafter, referred to as Patent Document 2). FIG. 21 illustrates the lighting device 60 in a state immediately before being subjected to glass sealing using a mold (see FIG. 3 of Patent Document 2). The LED light emitting element 61 is surface-mounted on a sub-mount 63 through bumps 62. The sub-mount 63 is connected to step portions formed in tip ends of leads 64A and 64B and covered with a sealing member 65. Glass is used for the sealing member 65. The sealing member 65 made of glass is formed so as to be thin on a lower side of the leads 64A and 64B and to be convexly thick on an output side of light emitted from the LED light emitting element 61.
In the entire lighting device 60, the LED light emitting element 61 is surrounded by a transparent glass portion and a metal portion which each have a thermal expansion coefficient in a range of 150% to 500% of that of the LED light emitting element 61. The feeding members (leads 64A and 64B) and the sealing member 65 are made larger in thermal expansion coefficient than the LED light emitting element 61 or the sub-mount 63. Therefore, a stress direction may be adjusted to prevent the occurrence of cracks due to a thermal shrinkage difference.
A method of manufacturing the lighting device 60 is as follows. A thin glass sheet 68, the sub-mount 63 on which the LED light emitting element 61 is mounted, the two leads 64A and 64B electrically connected to the sub-mount 63, and a thick glass sheet 67 located above the sub-mount 63 are set between an upper mold 71 having a semicircular recessed portion 71A formed on a surface and a lower mold 72 having a recessed portion 72A with a flat bottom. Then, while the glass sheets 67 and 68 are softened by heating at 450° C. in a vacuum atmosphere, the upper mold 71 and the lower mold 72 are moved in directions indicated by arrows to press the glass sheets 67 and 68. As a result, the glass sheets 67 and 68 are formed into the same dome shape as the sealing member 65 illustrated in FIG. 20.
However, when the conductive resin is filled into the through holes as described in Patent Document 1 and hardened by heat treatment to form the through-electrodes, it is difficult to maintain airtightness because of shrinkage during hardening. Moreover, the LED light emitting element generates heat during light emission. Therefore, when the LED light emitting element is repeatedly turned on and off, a temperature cycle occurs in which a temperature is repeatedly increased and decreased, and hence expansion and shrinkage are repeated. As a result, the airtightness of the interface between glass and the through-electrodes reduces, and hence moisture enters from the outside, to thereby shorten the life of the LED light emitting element.
In Patent Document 1, the conductive resin or solder is filled into the through holes and hardened to form the through-electrodes. A conductor film is further deposited by sputtering or evaporation and a rear surface electrode pattern is formed by a photo process using a photo mask. As a result, the number of manufacturing steps increases, and hence a manufacturing cost becomes higher.
In the LED light emitting element described in Patent Document 2, the sealing member 65 located on the light emitting surface side is warped to be in a dome shape, or a convex shape, and hence light emitted from the LED light emitting element 61 is diffused to all directions. Therefore, the light emitted from the LED light emitting element 61 may not be condensed in an upward direction or provided with directivity in the upward direction, and hence the emitted light may not be effectively used. In the lighting device 60, the thermal expansion coefficient of the sealing member 65 is adjusted to a value larger than the thermal expansion coefficient of the LED light emitting element 61 or the sub-mount 63 such that an internal stress based on the thermal expansion coefficient difference becomes a compression stress toward the center of the LED light emitting element 61, to thereby prevent cracks from occurring in the glass material. Thus, when the shape of the sealing member 65 sealing the LED light emitting element 61 is to be changed to, for example, a recessed shape to emit light having directivity, the compression stress toward the center is unbalanced, and hence it is likely to cause cracks to reduce reliability.
In the manufacturing method described in Patent Document 2, the sealing member made of glass is softened to seal the LED light emitting element 61, and hence the LED light emitting element 61 is exposed to a high temperature equal to or higher than, for example, 450° C. When the LED light emitting element is connected to wires by wire bonding, the wires are crushed by the glass because the softened glass has a high viscosity. When phosphors are to be dispersed in the sealing member 65 to convert a wavelength of light emitted from the LED light emitting element into another wavelength, the type of usable phosphor is limited because of the high temperature. Moreover, it is difficult to uniformly disperse phosphors in high-temperature and high-viscosity glass, and hence a desired effect may not be obtained. Therefore, there is a problem that the structure of the LED light emitting element or the mounting structure is limited.